"Sludge" is not raw human waste, but a by-product of wastewater or sewage treatment. Under normal treatment, raw human waste, carried in solution and suspension, is first subjected to primary treatment by settling and screening to dispose of sand, grit and some settleable solids. The settled solids are undigested or "raw" sludge. The layer containing the raw sludge is normally pumped to an anaerobic digester where the raw sludge is converted to an organically stable form.
The partially clarified sewage or supernatant, called "settled sewage," is then subjected to secondary treatment where it is used as nutrients by microorganisms that are provided with an excess of oxygen utilizing any one of many well-known methods of aeration. Typically, the secondary treatment comprises aeration followed by sedimentation ("settling") and digestion of the layer including the settled solids, aerobically and/or anaerobically. The resulting biological solids and treatment additives are settled out and the clarified effluent is removed for further processing. The material that settles out is digested and organically stable sludge. Although the sludge is typically separated by settling or sedimentation, it is primarily composed of a relatively small amount of solid particles, on the order of about one to about seven percent by weight, suspended in a liquid, primarily water. This product of the above-described process, or of an equivalent process, is what is treated by the process of the present invention.
As the sewage treatment process is conducted, the sludge accumulates in the waste treatment facility. This build-up of sludge must be periodically removed from the treatment plant to allow proper operation to continue. This accumulated sludge presents major disposal difficulties in the field of wastewater treatment.
Historically, sludge has been used as landfill or has been dumped in the ocean. Such means of disposal are no longer permitted due to health and environmental concerns. Major efforts are currently underway in the area of sludge management to provide appropriate means for disposing of sludge without harming the population or the environment. A wide variety of techniques have been and are currently being developed for treating raw sludge to allow for disposal, for example, by dewatering and subsequent incineration. In addition, pathogen reduction methods, such as the aerobic and anaerobic digestion techniques described above, have been incorporated into normal wastewater treatment processes in order to prepare the raw sludge for disposal, for example, by incineration. However, incineration introduces problems of air pollution from the burning of the sludge. These difficulties associated with the disposal of sewage sludge have spurred research efforts into new wastewater treatment and disposal techniques.
Resource recovery is a relatively new perspective for addressing the sludge disposal problem. Land application of human waste and sludge for their nutrient value to crops was common prior to the development of commercial chemical fertilizers and remains a common practice throughout many parts of the world. It is known that sludge can be used as a low grade fertilizer (in view of its low nitrogen content) or soil conditioner. It is also known that sludge can be used in the reclamation of desert lands. Applying sludge to sand dunes, for example, has been shown to be effective in stopping the growth of the dunes. The use of sludge in the reclamation of deserts, and as an agricultural fertilizer, is described in the published report entitled "Sand Dunes: Understanding Their Control", by George D. Ward and Associates, prepared for the U.S. Department of the Navy under Contract No. N62474-73-C-5136, dated 1971-1974.
However, current concerns for public health require that sludge be further processed if it is to be used as a fertilizer, as a soil conditioner, in land reclamation projects or the like. The sludge which is the current product of sewage treatment plants cannot be used for agricultural purposes without further treatment. Although sewage sludge is not as severe a threat to health or the environment as raw sewage, it still contains substantial quantities of bacteria, viruses and other organisms. Pasteurization, when applied to the pretreated sludge, is one of the methods considered to be acceptable to kill all harmful organisms. The publication entitled "Sludge Pasteurization Systems", of Zimpro Incorporated, dated May, 1978, discussed the need for pasteurization of sewage sludge. This publication discloses a reactor which sufficiently heats the sludge to cause pasteurization at up to 92.8.degree. C. (200.degree. F.) followed by digestion. The current U.S. federal standard for pasteurization of sewage sludge is to heat the sludge to 70.degree. C. (158.degree. F.) for at least thirty minutes. By definition, pasteurization will kill all pathogenic organisms remaining in the sludge.
Pasteurized sludge is suitable for disposal in land fills, for land reclamation, or for other agricultural purposes, and is the product produced by the present invention.
The present invention comprises a process for treating sewage sludge while in transport in large quantities in essentially conventional tanker vehicles, such as in ships, railroad cars, trucks, and the like, such that the sludge is fully pasteurized and capable of being safely used in agriculture, land reclamation or the like. Preferably, in accordance with the present invention, waste heat from the vehicle's engine used to propel the vehicle is used as a heat source to pasteurize the sludge.
The transport of sludge in tanker vessels was considered in a report sponsored by the U.S. Environmental Protection Agency, under Grant No. C-06-1042-011. The report concludes that the exportation of sludge as waste in tankers for the distance and under the circumstances of the report is not economically practical. However, this report does not discuss or even recognize the possibility or desirability of pasteurizing sludge while it is being transported in the tanker vessels.
Various patents disclose devices and methods for treating raw sewage in seagoing vessels. Examples are U.S. Pat. Nos. 3,337,448 3,395,799, 3,535,234, 3,807,564, 3,821,107 and 3,992,299. However, most of these references deal only with processing of small quantities of raw sewage generated by the crews aboard the ships. They do not concern the transportation and later use of treated sewage sludge. U.S. Pat. No. 3,395,799 discloses a device which heats sewage on a ship using the waste heat from the ship's engines. However, the patent also proposes dumping the heated sewage into the ocean and does not address transporting the sludge to another location for further use, such as land or agricultural applications.
The present invention is a novel method which allows sludge to be pasteurized, and therefore, conveniently and economically converted into a useful product while being transported. Particularly in its preferred form, the method is more efficient and economical, since some or all of the required process heat is obtained from the waste heat of the transportation vehicle's propulsion system. Large quantities of sewage sludge can be pasteurized effectively and completely while transporting the sludge long distances. In accordance with the present invention, not only is sewage sludge used for beneficial purposes, such as in land reclamation or agriculture, but also sludge is economically and practically disposed of without the health dangers previously associated with the disposal or use of sewage sludge.